The noise FIG. F of a receiver varies as a function of the admittance Y.sub.S =G.sub.S +jB.sub.S of its input termination, or "source admittance," according to the equation ##EQU1## where F.sub.min is the minimum, or optimum, noise figure achievable through adjustment of the source admittance, Y.sub.opt =G.sub.opt +jB.sub.opt is the source admittance at which the minimum noise figure is produced, and R.sub.n is a positive parameter having the dimensions of resistance. The noise performance of a receiver may be characterized by specifying the four noise parameters F.sub.min, R.sub.n, and Y.sub.opt =G.sub.opt +jB.sub.opt. It is well known that equation (1) may equivalently be written in terms of source impedance, source reflection coefficient, or other physically equivalent source quality. For further information concerning noise figure refer to "IRE Standards on Methods of Measuring Noise in Linear Twoports, 1959," Proceedings of the IRE (Jan 1960), pp. 60-68.
A receiver's noise parameters can be determined by various methods. One method entails determining the receiver's noise figures for various known source admittances, searching for the minimum noise figure F.sub.min. Once F.sub.min has been experimentally identified, the noise parameters F.sub.min and Y.sub.opt =G.sub.opt +jB.sub.opt are known. A measurement of the receiver's noise figure at another known source admittance allows the noise parameter R.sub.n to be calculated.
This method of determining a receiver's noise parameters by searching for F.sub.min has numerous drawbacks. It requires the input termination to be able to match the source admittance Y.sub.opt at which the minimum noise figure F.sub.min is produced, requiring an input termination which is finely tunable over a wide range of admittances. The accuracy of the determined noise parameters is dependent on accurately locating the source admittance Y.sub.opt at which the minimum noise figure is produced. If the bottom of the surface defined by equation (1), given above, has relatively little curvature, then locating Y.sub.opt is difficult. Also, if the surface defined by equation (1) has steep slopes near the minimum noise figure, the minimum noise figure F.sub.min may be inaccurate. This searching method can require considerable time.
Noise figure is not directly measurable, but can be calculated from two noise power measurements taken at different noise source temperatures according to the equation ##EQU2## where ENR is the excess noise ratio and Y is the ratio P.sub.HOT /P.sub.COLD where P.sub.HOT and P.sub.COLD are measured noise powers at HOT and COLD source noise temperatures, respectively. The accuracy of the resulting noise figure depends on the total receiver gain remaining constant between measurements of P.sub.HOT and P.sub.COLD, which requires that the source admittance Y.sub.S not change with the effective temperature of the source. Thus, a HOT source admittance must equal the respective COLD source admittance over a wide frequency band. In addition, the effective ENR of the source at the input of the receiver must be known for all the measured source admittances. Furthermore, if, as is common, an adjustable two-port tuner is used between the noise source and the receiver input to set the source admittance to various values, its loss must be known for each of the settings used in the noise power measurements.
Lane describes a technique for determining the four noise parameters from four or more noise figure determinations. Richard Q. Lane, "Determination of Device Noise Parameters," Proceedings of the IEEE, Vol. 57, (Aug. 1969), pp. 1461-62. Through the use of more than four noise figure determinations, and a fitting procedure, such as least-squares analysis, the noise parameters can be found while minimizing the effects of random measurement errors. However, like the previously described method, this technique requires multiple determinations of noise figure, with its associated measurement problems.
Another method of determining a receiver's noise parameters described by Adamian and Uhlir in "Simplified Noise Evaluation of Microwave Receivers," IEEE Transaction on Instrumentation and Measurement, Vol. IM-22, No. 2 (June 1973), pp. 181-82, requires only one noise figure measurement at an arbitrary source admittance and a plurality of noise power measurements for various other source admittances. Thus, any problems associated with determining noise figure are decreased in that only one noise figure need be determined. Unfortunately, the problems still exist for that one determination.
What is needed, then is an improved method for accurately determining a receiver's noise parameters without having to determine its noise figure for any source admittance.